http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56175
--- Comment #7 from Yuri Rumyantsev <ysrumyan at gmail dot com> 2013-02-12 13:05:16 UTC --- (In reply to comment #6) > (In reply to comment #5) > > This pattern is already recognized by simplify_bitwise_binary but only for > > usual int type, i.e. if we change all short types to the ordinary int (or > > unsigned) this simplification takes place (dump after 1st forwprop): > > > > <bb 4>: > > x_8 = x_2(D) >> 1; > > y_9 = y_4(D) >> 1; > > _10 = x_8 & 1; > > _11 = y_9 & 1; > > _16 = x_8 ^ y_9; > > z_12 = _16 & 1; > > > > i.e. the issue is redundant type conversions: > > > > <bb 3>: > > x_7 = x_2(D) >> 1; > > y_8 = y_4(D) >> 1; > > _13 = x_7 & 1; > > _9 = (signed char) _13; > > _14 = y_8 & 1; > > _10 = (signed char) _14; > > _11 = _9 ^ _10; > > > > I assume that if we delete these redundant conversions the required > > simplification will happen. > > Ah, well. The issue is that we transformed (unsigned char)y & 1 > to (unsigned char)(y & 1). Hi Richard, We'd like to fix this issue since we can get +10.5% speedup on Atom. What is your opinion on how better to fix this issue with 1st pattern in simplify_bitwise_binary? I have no idea why gcc does such transformation and what gain we can get from it - decrease size of constant or create more opportunities for cse? I can propose the following possible changes: 1. Introduce a hook for doing such transformation. 2. Introduce a new forwprop pass that does not do such transformation. 3. Do not perform such transformation for small positive constant. 4. Do not performa such transformation if (type-x) c == c. etc. Any help will be appreciated. Yuri.
